The dry sandy soils found in wheat growing regions, such as those extending from central Kansas down into Texas, wear and dull the outer edges of cultivating disc, coulter and seed drill blades used in these regions. With the major wear occurring at the outer edges of the cultivating blades used in these dry sandy soils, they require frequent re-edging and re-sharpening. Re-edging and re-sharpening by techniques such as grinding undesirably removes metal, and reduces hardness due to heating during grinding; thereby drastically shortening the useful life of the blades. There are sharpening machines in the market place that roll and pinch the edges of cultivating blades to a re-edged and re-sharpened condition. Traditional heat quenched boron steels and water quenched steels cannot be rolled due to the quenched condition of these steels, which gives them high Rockwell C hardness greater than about 47 RWC. Typically, boron steels are heated to about 1500° F. at the start of the heat quench curve. Quenching is done using water, water plus oil, and glycol based quench liquids. The inventive heat treatment involves light tempering for about one hour. For the inventive heat treatment, the cultivating blades formed from the heat quenched boron steels should not be stacked or touching each other so that circulation is completely around each blade, so that the full extend of the heating to reduce the stress caused by quenching in the edge can be evenly distributed. Boron steels useful in the invention should have a minimum of from about 0.018% to a maximum of about 0.039% carbon; and, from about 0.60% to a maximum of about 1.51% manganese. Boron gives the steel enhanced harden ability as compared with non-boron steels.
The inventor has discovered that to make cultivating blades formed from quenched boron steel into “rollable” products, they must be heat treated in a certain inventive manner. The inventive heat treatment reduces the molecular stress in the blades and reduces the Rockwell C hardness to within a range wherein the blades may be re-edged and re-sharpened using known rolling machines. The inventive heat treatment includes heating the cultivating blades for a time and temperature which reduces the Rockwell C hardness to between 40 and 46 RWC, preferably between about 41 and 45 RWC, and more preferably between about 42 to 44 RWC; whereby the inventive blades can be roller re-edged and re-sharpened, yet retain excellent toughness, hardness and wear characteristics. The inventor has discovered that if the Rockwell C hardness is reduced below 40, the wear ability of the cultivating discs is significantly reduced. The desired Rockwell C hardness of the heat quenched boron steel used in the present invention is between about 47 to 60, preferably 47 to 55, and more preferably between 47 and 50. If the Rockwell C hardness runs higher, then increased treatment temperatures are required; the extra heat will lessen the wear characteristics of the product blades.
Wear in cultivating blades used in wetter and less sandy soils, differs from the wear in blades used in the dry, sandy soils. In wetter and less sandy soils, the wear tends to be more evenly spread over the entire surface of the blades, and the blades tend to be “self-sharpening”. Note U.S. Pat. No. 7,143,838, anti-penultimate paragraph on page 7. The inventive cultivating blades can be repeatedly re-edged and re-sharpened using rollers. Additionally, the inventive cultivating blades treated according to the present inventive method exhibit the unexpected and beneficial properties of having edges that withstand impact to a high degree, and do not chip easily.
The safety and durability of rotary cultivating blades is of primary concern in the production thereof. Experience with blade failures has driven manufacturers to provide blades with low hardness to prevent impact failures. However, this has led to failures due to bending and excessive wear. Blades with high Rockwell C hardness, such as between 47 and 60, are subject to unacceptable impact failure, and cannot be roller sharpened without damage to the sharpening rollers. High carbon steels exhibit higher levels of hardness, but are difficult to work, have reduced toughness, and result in accelerated tool wear, adding to manufacturing costs. Mitigating the benefits of high hardness is the tendency to suffer unacceptable catastrophic impact failure. Heat quenched boron steels, such as heat quenched grade 10B38, exhibit desirable high levels of toughness and have been used for lawn mower blades, as well as various cultivating blades. However, the high hardness of the heat quenched boron steels makes roller sharpening impractical. Blades made from untreated boron steels are susceptible to wear at a greater than optimum rate, and have less than optimal resistance to edge deformation, bending and fatigue. The hardness of boron steels is usually elevated by marquenching or other quench treatments to a Rockwell C hardness somewhere between 47 and 60. Typically, quenching is done using water, or water plus oil, or glycol base quench liquid. See, for example, U.S. Pat. No. 5,916,114, and U.S. Pat. No. 5,899,052. In the '114 patent, the boron steel is heat quenched to a Rockwell C hardness between 48 and 55. The addition of more soluble oil or glycol to the quench water slightly reduces the quench hardness, and somewhat reduces cracking.
Rockwell C hardness (RWC) is used to measure the hardness of hard steels, such as boron steels. In the Rockwell test, a hardened-steel ball or a diamond cone is used. Various indenters and loads are employed for different materials. A minor load, which does not deform the metal, is used to seat the indenter, and a major load is applied for indentation. The depth of the penetration effected by the major load is a measure of Rockwell hardness. The Rockwell C scale is used for hard steels, such as boron steels. For the Rockwell C scale, a diamond cone is used with a load of 150 kilograms. Rockwell tests are easily and rapidly made and are widely used in industry. See “Metallurgy For Engineers”, Wulff, Taylor and Shaler, New York, John Wiley & Sons, Inc. 1952 (Fifth Printing: June, 1960), pp. 210-212, which is incorporated herein by reference in its entirety.
Boron steels are available in a wide range of formulations which typically include carbon, phosphorus, silicon, copper, chromium, titanium, manganese, sulfur, aluminum, nickel, columbium or other rare earths, molybdenum, tin, calcium, zirconium, and boron, and the balance iron (note U.S. Pat. No. 4,362,553, third paragraph in “SUMMARY OF THE INVENTION”; also, U.S. Pat. No. 4,440,568, which is incorporated herein by reference in its entirety). The boron is present to increase the harden ability of the steel. To develop the maximum harden ability effect, boron must be present in the steel in a minimum amount of about 0.0005% boron. Greater than 0.003% boron has little or no beneficial effect and is wasted. Since boron has a strong affinity for oxygen and nitrogen, these elements must either be removed or controlled for boron to have its full hardening effect. It has been a practice to add boron to steel with titanium and zirconium present to protect the boron from nitrogen, and with aluminum present to protect the boron from oxygen. Alloying additives are commonly used containing mixtures of small amounts of boron, and more substantial amount of titanium, and rare earths to protect the boron from nitrogen and oxygen. The titanium is also present in the boron containing additive to promote correct mixing with the molten steel. The amount of titanium is determined by each steel mill, as every alloying process differs slightly. The product boron steel will normally contain on the order of 0.0005% to about 0.003% residual boron, and on the order of about 0.035% to about 0.055% residual titanium with good harden ability effect. The removal or addition of zirconium or aluminum must to done with great care. As noted above, typically the boron steels are quenched, using water, water plus oil, or glycol base quench liquids, to a Rockwell C hardness of between a minimum of about 47 and a maximum of about 60. Where the boron steels are water quenched, temperatures of about 1500° F. are required when the quench curve is started.
The heat quenched boron steels useful in this invention range from about grade 10B21 through about grade 15B35. They can have a heat quenched hardness ranging from a about 46 RWC as a minimum to about 60 RWC as a maximum. In a preferred embodiment of the present invention, grade 15B25 boron steel is used having a heat quenched Rockwell C hardness of about 47 to 50. A most preferred and best boron steel for use in the present invention is grade 15B26. Osmundson Specification boron steel 15B26 contains about 0.24 to 0.28% carbon and about 1.0 to 1.3% manganese. It also contains 0.015 maximum % phosphorous; 0.15 to 0.03% silicon; 0.15% copper; 0.10 to 0.20% chromium; 0.0005 to 0.003% boron; 0.03 to 0.06% titanium; 0.012% nitrogen; 1.10 to 1.25% manganese; 0.010 maximum % sulfur; 0.015 to 0.035% aluminum; 0.12 maximum % nickel; 0.008 max % columbium; 0.015% tin; and 0.001 to 0.005% calcium. (Osmundson Tillage Tools, Osmundson Mfg. Co., Box 158, Perry, Iowa 50220, “www.osmundson.com”). Examples of boron steels useful in the present invention, on either side of grade 15B26 boron steel, are grades 15B23, 15B27 and 15B31. The compositions of various other examples of boron steels useful in the present invention are described in a listing by Sumitomo Metals (Kokura), Ltd. Note their website (http://www.kokura.sumitomometals.co.jp/e/product/sumitomo/010 . . . ), which web pages are copied and identified in the IDS of this application, and which web pages are incorporated herein by reference in their entirety. Grade 10B21, for example, is described as a low carbon boron steel containing about 0.18 to 0.23% carbon; 0.15 to 0.30% silicon; 0.80 to 1.10% manganese; less than or equal to 0.040% phosphorous; less than or equal to 0.050% sulfur; and, at least 0.0005% boron. Grade 10B22 is described as a low carbon boron steel containing about 0.18 to 0.23% carbon; 0.70 to 1.00% manganese; less than or equal to 0.040% phosphorous; less than or equal to 0.050% sulfur; and, at least 0.0005% boron. Grade 15B25 is described as a low carbon boron steel containing about 0.22 to 0.30% carbon; less than or equal to 0.10% silicon; 0.75 to 1.25% manganese; less than or equal to 0.035% phosphorous; less than or equal to 0.040% sulfur; and, at least 0.0005% boron. Grade 15B25 boron steel can be heat treated at a low temperature of about 450° F. to reduce embrittlement. However, the Rockwell C hardness of the 15B25, would not be reduced from the original RWC of 47 to 50; and, use of this low a temperature would not be recommended, due to adverse affect of that high level of hardness on any roller equipment used in attempting roller re-edging or sharpening. Grade 15B26 is described as a low carbon boron steel. Grade 10B35 is described as a carbon boron steel containing about 0.32 to 0.38% carbon; 0.15% to 0.35% silicon; 0.60 to 0.90% manganese; less than or equal to 0.030% phosphorous; less than or equal to 0.035% sulfur; less than or equal to 0.35% chromium, and, at least 0.0005% boron. Grade 10B37 is described as a carbon boron steel containing about 0.33 to 0.39% carbon; 0.15% to 0.35% silicon; 0.70 to 1.00% manganese; less than or equal to 0.030% phosphorous; less than or equal to 0.035% sulfur; less than or equal to 0.35% chromium, and, at least 0.0005% boron. Grade 15B35 is described as a carbon boron steel containing about 0.34 to 0.39% carbon; 0.15% to 0.35% silicon; 0.95 to 1.10% manganese; less than or equal to 0.030% phosphorous; less than or equal to 0.035% sulfur; less than or equal to 0.35% chromium, and, at least 0.0005% boron. Grade 10B36 is described as a carbon boron steel containing about 0.32 to 0.37% carbon; 0.15% to 0.35% silicon; 1.20 to 1.50% manganese; less than or equal to 0.030% phosphorous; less than or equal to 0.035% sulfur; and, at least 0.0005% boron.
U.S. Pat. No. 5,899,052 illustrates making rotary blades from boron steels, such as boron steel 10B38, which blades are heat treated to yield a Rockwell C hardness of 48 or above. These elevated hardness would preclude roller sharpening. Boron steel 10B38 contains iron with about 0.035-0.42% carbon, 0.60-0.90% manganese, and standard limits of sulfur and phosphorus. The “B” means that is has been boron treated to improve harden ability, and can be expected to contain 0.0005-0.003% boron. Not hardness, but harden ability, which is an important difference. See “http://www.finishing.com/93/98.shtml”, letter 9388. U.S. Pat. No. 5,916,114 similarly forms rotary blades from boron steel, such as grade 10B38 alloy, which has been quenched to a Rockwell C hardness between 48 and 55. Boron steels having a hardness above 47 RWC cannot be re-edged and sharpened by rolling due to their excessive hardness and brittleness. They tend to break, crack or otherwise fail during roller sharpening; and, due to their extreme hardness damage rollers used in roller sharpening. Typically, disc, coulter and seed drill blades used in normal less sandy, wetter soils tend to be self sharpening (note U.S. Pat. No. 7,138,838, for example).
The boron steels useful in the present inventive method range from grade 10B21 to 15B35, which have a quenched hardness ranging from a minimum of about 47 RWC to a maximum of 60 RWC; preferably from about 47 to 55 RWC; and, most preferably from about 47 to 50 RWC. Grade 15B26 boron steel with a heat quenched Rockwell C hardness ranging from 47 to 50 constitutes a best mode for use in this invention. The widest range of tempering temperature, especially for heat quenched grade 15B26 boron steel, is from about 710 to 730° F. This will give a Rockwell C hardness of about 41 to 45. The preferred heating temperatures and times for heat treating and relieving the stresses in cultivating blades formed from heat quenched 15B26 grade steel are from about 715 to 725° F. for about 45 to 55 minutes. This will reduce the RWC to about 42 to 44. The 15B26 boron steel will normally have a carbon content ranging from about 0.24 to 0.28; and, a manganese content ranging from about 1.00 to 1.50%. Within the range of 10B21 to 15B35, and on either side of 15B26, are, as examples, grades 15B23, 15B27 and 15B31 boron steels. The quench heat treatment of 15B26 requires the steel be heated to about 1500° F., when it starts the quench using straight water. This would be a best mode to insure achieving the narrower most preferred hardness range for the inventively heat treated blades of 42 to 44; and, would result in heat treated product blades exhibiting optimal wear and optimum roll-ability. The sufficient time and temperature ranges for all the boron steel grades covered by the present invention is much greater. For 10B21 a temperature of 350° F. for 30 minutes would be adequate. At the other end of the range of useful boron steels, 15B35, for example, requires about one hour at a temperature of 825° F. These temperatures can vary slightly depending on the amount of carbon in the boron steel. For instance, 15B35 at its maximum can have about 0.38 to 0.39% carbon, requires a heat treatment temperature of about 850° F. At its minimum of about 0.32 to 0.33% carbon, 15B35 requires a heat treatment of about 785° F. The heat treatment of the formed blades in the present inventive method constitutes a light tempering, typically for about one hour, in a continuous furnace, with circulation completely around each formed blade. During the inventive heat treatment, the blades should not be stacked or touching each other, so that the full extend of the heating to reduce the stress caused by quenching can be evenly distributed. In the present invention the useful boron steel, which range from grade 10B21 to grade 15B35, can have an as quenched Rockwell C hardness ranging between a minimum of about 47 RWC to a maximum of about 60 RWC. The perfect hardness for grade 15B25 is from about 47 to about 50 RWC. The inventive heat treating method results in the inventive cultivating blades having edges that can be roller re-edged and re-sharpened, do not chip easily, and can withstand impact to a substantially high degree.
The prior art disc, coulter and seed drill blades are typically made from boron steels which have been quenched to such a high hardness that roller sharpening is not feasible. Also, the high quenched hardness, e.g., above 47 RWC, can lead to impact failures due to brittleness, during use such as in rocky soil. Attempts to avoid brittleness by reducing the hardness have resulted in products which quickly wear out. Boron steels, however, combine characteristics of hardness, wear resistance and strength, along with corrosion and oxidation resistance that are highly desirable. There has long been a need in the prior art for cultivating disc, coulter and seed drill blades made of boron steels, which can be roller re-edged and re-sharpened without damage to the rollers, which do not chip during roller hardening, which can withstand impact to a high degree, and which retain excellent hardness, strength, toughness, and wear resistance. The present inventors have discovered a method for heat treating blades made from heat quenched boron steels, which results in inventive cultivating blades which exhibit these characteristics.